Field
The invention relates to a sensor attachable to a body of a user. More particularly, the invention relates to transmitting and/or receiving electromagnetic energy by the sensor.
Description of the Related Art
Lately technology has provided us with small-sized radio devices or personal digital assistants (PDA). One example of such is a wrist computer which is a relatively small apparatus used by a person on his/her wrist. The size of the wrist computers creates certain challenges for the design thereof. One such challenge is the miniaturization of an antenna to be mounted on it.
A resonator-type antenna is often used in small-sized radio devices. The antenna-resonator is a device that resonates and radiates energy. In other words it oscillates electromagnetic waves at certain frequencies, called its resonance frequency, with greater amplitude than at other frequencies and the oscillation is based on the dimensions of the resonator that are integral multiples of the wavelength at the oscillating frequencies.
The small size prevents the use of ordinary resonator antennas where the substrate material is the air because of relatively big volume requirement. Compression of this type of antenna causes the bandwidth of the antenna to be small, increases the carrier frequency of the electromagnetic radiation and limits the gain of the antenna-resonator. Consequently, it is common to replace the air as a substrate with a dielectric material. Since air is replaced with the dielectric material, the physical size of the antenna can be decreased in proportion to the dielectric constant of the substrate material of the resonator. This property is also known as the permittivity. In ordinary air isolated antennas this property cannot be utilized. Further, the wave propagation velocity is somewhere between the speed of radio waves in the substrate, and the speed of radio waves in air.
By adjusting the permittivity of dielectric substrate, the physical length of the resonator may be decreased. Regardless of the decrease in the physical length, the electrical length of the resonator may be the same. In general, the electrical length is different from the physical length. However, they are proportional to each other via the dielectric constant. This is because the electrical constant affects the propagation velocity in the substrate, and the electrical length is the physical length multiplied by the ratio of the propagation time of an electromagnetic signal through the substrate to the propagation time of an electromagnetic wave in free space over a distance equal to the physical length. From another point of view, it can be said that since the air as is replaced with the dielectric material, the electrical length of the resonator can be increased while keeping the physical size constant.
One way to increase the effective size of a radiation structure is to include the ground plane as a part of the antenna structure by enabling the so called “ground plane resonance”. Generally, this is achieved by forming an electric field between the open end of the antenna resonator and the ground plane. This electric field enables surface current on the ground plane leading to a ground plane resonance. However, the electric field type of coupling is weak and, therefore, the gain of the radiation structure is low.
As the antenna of the wrist computer is generally the first receiving component in the receiving chain, its role is certainly significant. The transmitter may be, e.g., a belt worn by a person around his/her chest and capable of observing the heart beat rate. Thus, it is important to make the gain of the antenna as high as possible. Since the size of the apparatus prohibits the use of a larger single antenna that could increase the gain, other solutions to improve the antenna gain are needed.